Answers

Shield (middle mouse): Stoneskin. An aura that protects you from damage, but makes you vulnerable to knockdown. Will drain steadily, but can be restored by Life-based healing effects and the Staff of Life.

Life + Shield (rightclick): Lifemine. Creates an arc (shift-rightclick creates a circle) of landmines that grant a large health boost to anyone or anything that steps on them. This works on enemies, too.

(Any) + Arcane (right click) = (Element) Ray. A beam attack that will shred any single mob in short order. Can be fired continuously, but turning to aim is slow.